Rated 5 out of 5 stars

I recently learned about this new API
https://github.com/thundernest/addon-developer-support/wiki/Using-the-WindowListener-API-to-convert-a-Legacy-Overlay-WebExtension-into-a-MailExtension-for-Thunderbird-78

which is meant to ease the migration of legacy addons to TB 78.
Maybe this is helpful for you...

This user has a previous review of this add-on.

Hi, I've seen the API, but even with the API it requires a significant amount of work, because of the backwards compatibility I had and the tight integration with Thunderbird with the UI and Options.
I'm working on it…